Lead-acid batteries, which are used in motor vehicles with start/stop function and are also used in micro-hybrid motor vehicles, are prone to build up of sulfate layers, particularly when they are operated with only a partial charge. Lead-acid batteries require a refreshment or desulfation cycle at predetermined intervals in order to remove sulfate layers. For efficient refreshment, it is typically necessary for the lead-acid battery to be chanted for a continuous period, preferably 24 hours, at a maximum voltage, i.e. 15V. However, such extended charging periods cannot be ensured when the duration of a journey is typically much shorter, on the order of twenty to thirty minutes. In addition, operation of the motor vehicle at the necessary high voltage for the extended period of time necessary to properly achieve desulfation leads to an increase in fuel consumption of the motor vehicle.
DE 10 2011 054 582 A1 discloses, inter alia, a device for controlling a battery system, wherein a load secondary battery and a lithium secondary battery with higher power density and energy density relative to the lead secondary battery are used. Semiconductor switches in the form of MOSFETs are electrically conductively connected both to the alternator and to the lead secondary battery and the lithium secondary battery and are actuated via a controller (ECU) in such a way that the lithium secondary battery is charged close to a setpoint charge value daring the operation of the internal combustion engine. A variable setting unit sets this setpoint charge value in a variable manner on the basis of a state variable dependent on a regenerative state of charge and/or state of discharge of the lithium secondary battery.
EP 2 041 862 B1 discloses, inter alia, an electrical energy source and a method for controlling an electrical energy source, wherein a high-voltage battery in the form of a lithium-ion battery and a low-voltage battery in the form of a lead-acid battery are also used.
There is a need for a method for operating an on-board power supply system of a motor vehicle in such a manner that enables an effective refreshment or destination of a primary battery while saving as much fuel as possible.